Printing machines are known, such as for example the one described in document U.S. Pat. No. 3,528,620 A, having a rotary central impression drum provided with an outer surface suitable for supporting a web substrate between a substrate entry point located on an entry side of said central impression drum and a substrate exit point located on an exit side of the central impression drum. A plurality of printing groups are uniformly distributed along each of said entry and exit sides of the central impression drum, and each printing group has a printing cylinder depositing a respective water-based ink or solvent-based ink at a specific impression point on said web substrate supported on said central impression drum.
In these known printing machines, between each pair of adjacent printing groups there is arranged at least one drying unit configured for applying a drying agent, such as a drying air stream or infrared radiation, to the web substrate supported on the central impression drum and thereby drying by evaporating the water or solvent the ink deposited on the web substrate by the preceding printing group sufficiently to allow depositing the subsequent ink thereon without causing the inks to run, all the drying units being similar or having comparable dimensions. After the last printing group and the substrate exit point, with respect to the forward movement direction of the web substrate, there is arranged a final drying unit generally in the form of a drying tunnel applying a drying agent to the web substrate for final drying of all the inks deposited thereon.
Throughout this description, the term “ink” is generically used to designate inks, varnishes, adhesives, coatings and other water- or solvent-based compounds that can be applied to a substrate by means of a printing machine.
In known printing machines of the type described above, a problem arises when the first ink applied by the first printing group, with respect to the forward movement direction of the web substrate, is a base ink having a relatively high viscosity, generally white in color, applied as a relatively thick layer completely covering the surface of the web substrate or a significantly large area thereof, on which the rest of the inks will be successively applied, because the drying agent applied by the first drying unit in combination with a high speed of movement of the web substrate is insufficient to perform a drying or even a partial drying of said base ink which would allow the subsequent ink to be applied on the base ink without causing the inks to run.
A similar problem occurs when the ink applied by the last printing group is a transparent coating varnish or ink, generally having a shiny finish, applied as a layer completely covering the surface of the web substrate or a significantly large area thereof over the previously deposited inks, because the drying agent applied successively by all the drying units in combination with a high speed of movement of the web substrate is insufficient for partially drying the superimposed inks which would allow the subsequent application of the coating varnish or ink without causing the inks to run.
Conventional drying units commonly use a drying air stream as a drying agent, and comprise at least one air nozzle blowing drying air supplied from a drying air supply device, which generally includes an air supply turbine, and optionally a heater and a dehumidifier. Nevertheless, it has been found that even by increasing the temperature at which the drying air is heated and/or reducing the humidity of the drying air and/or increasing the flow rate at which the drying air is supplied to the drying units, conventional drying units do not provide sufficient drying to allow using a base ink applied by the first printing group and/or using a coating varnish or ink applied by the last printing group without the problems of inks running.
Printing machines are also known which use infrared radiation as a drying agent to evaporate water or solvent from the inks deposited on the web substrate.
An objective of the present invention is to provide a printing machine having a central impression drum provided with at least one extensive drying unit arranged between the first printing group and a subsequent printing group, with respect to the movement direction of the web substrate, which allows using a base ink applied by the first printing group and/or at least one extensive drying unit arranged between the last printing group and a preceding printing group, with respect to the movement direction of the web substrate, which allows using a coating varnish or ink applied by the last printing group.